load your map
 open the console (~ tilde key)
 bind a key to the screenshot command
 /bind F12 screenshot
 close the console
 hit the F12 key. A .tga file will be created. 
 If you bind the screenshotJPEG command to a key you will create a JPEG instead of a TGA file.  
 commands: 
 screenshot : save current viewport to a TARGA image file (usually named sequentially shot0001.tga) 
 screenshotJPEG : save current viewport to a JPEG image file (usually named sequentially shot0001.jpg)  
 See Commander Keen's Quake3 Arena Console Page (
http://www.planetquake.com/q3console/) for a complete list of Variables, Commands and Tweaking.